等离子体电解氧化过程中阴极微放电的证据
The Evidence of Cathodic Micro-discharges during Plasma Electrolytic Oxidation Process
摘要：
在含氟电解液中，采用双极性脉冲电流对EV 31镁合金进行了等离子体电解氧化处理。观察和研究了不寻常的PEO阴极微放电。结果表明，阴极微放电表现出集体间歇行为，讨论了在层/电解质和层/金属界面的电荷积累。光学发射光谱法用于确定电子密度（典型值）。10 15 cm-3）和电子温度（典型值）。7500 K），同时指出F-阴离子对阴极微放电的影响。等离子体电解氧化（PEO）是一种很有前途的等离子体辅助轻金属合金（如铝、镁、钛）表面处理方法。虽然PEO工艺使得生长具有有趣的耐腐蚀和耐磨性能的氧化物涂层成为可能，但涂层生长的物理机制尚未完全了解。通常，该方法包括在金属件和对电极之间施加高电压差，金属件和对电极都浸入电解质浴中。与阳极氧化相比，主要差异涉及电解质组成以及电流和电压范围，其在PEO 1中高至少一个数量级。电流和电压的这些显著差异意味着介电击穿，并且因此意味着在处理下的样品的表面上出现微放电。这些微放电被认为是形成介电多孔结晶氧化物涂层的主要贡献者。2然而，控制这些微放电出现的击穿机制仍在研究中。Hussein等人3提出了一种基于等离子体化学成分差异的三种不同等离子体形成过程的机制。Jovovi等人关于等离子体物理性质的结果4，5似乎证实了这一机制，并指出了等离子体组成中基底材料的重要性。6与直流导电PEO工艺相比，使用双极性脉冲直流或交流电流源通过电流波形参数提供了补充控制宽容度。这些参数对微放电行为的影响已经在以前的几个作品中进行了研究。2，3，7，8这些研究的主要结果之一是在阴极电流半周期内没有微放电。9-11即使阴极半周期对PEO的效率以及涂层的生长和组成有明显的影响，微等离子体也只出现在阳极半周期。Sah等人8已经观察到氧化物层的阴极击穿，但是在非常高的电流密度（10 kA·dm-${}^2$）下，并且在样品制备的几个步骤之后。交流电流中微放电现象的几种模型已经被提出。1，2，8，12，13虽然在通常的工艺条件下从未观察到阴极微放电，但本研究旨在确定促进阴极微放电的合适条件，并研究这些微等离子体的主要特性。
英文摘要：
Plasma electrolytic oxidation (PEO) processing of EV 31 magnesium alloy has been carried out in fluoride containing electrolyte under bipolar pulse current regime. Unusual PEO cathodic micro-discharges have been observed and investigated. It is shown that the cathodic micro-discharges exhibit a collective intermittent behavior which is discussed in terms of charge accumulations at the layer/electrolyte and layer/metal interfaces. Optical emission spectroscopy is used to determine the electron density (typ. 10 15 cm-3) and the electron temperature (typ. 7500 K) while the role of F-anions on the appearance of cathodic micro-discharges is pointed out. Plasma Electrolytic Oxidation (PEO) is a promising plasma-assisted surface treatment of light metallic alloys (e.g. Al, Mg, Ti). Although the PEO process makes it possible to grow oxide coatings with interesting corrosion and wear resistant properties, the physical mechanisms of coating growth are not yet completely understood. Typically, the process consists in applying a high voltage difference between a metallic piece and a counter-electrode which are both immersed in an electrolyte bath. Compare to anodizing, the main differences concern the electrolyte composition and the current and voltage ranges which are at least one order of magnitude higher in PEO 1. These significant differences in current and voltage imply the dielectric breakdown and consequently the appearance of micro-discharges on the surface of the sample under processing. Those micro-discharges are recognized as being the main contributors to the formation of a dielectric porous crystalline oxide coating. 2 Nevertheless, the breakdown mechanism that governs the appearance of those micro-discharges is still under investigation. Hussein et al. 3 proposed a mechanism with three different plasma formation processes based on differences in plasma chemical composition. The results of Jovovi{ć} et al. 4,5 concerning physical properties of the plasma seem to corroborate this mechanism, and also point out the importance of the substrate material in the plasma composition. 6 Compared with DC conducted PEO process, using a bipolar pulsed DC or AC current supply gives supplementary control latitude through the current waveform parameters. The effect of these parameter on the micro-discharges behavior has been investigated in several previous works. 2,3,7,8 One of the main results of these studies is the absence of micro-discharge during the cathodic current half-period. 9-11 Even if the cathodic half-period has an obvious effect on the efficiency of PEO as well as on the coating growth and composition, the micro-plasmas appear only in anodic half-period. Sah et al. 8 have observed the cathodic breakdown of an oxide layer but at very high current density (10 kA.dm-${}^2$), and after several steps of sample preparation. Several models of micro-discharges appearance in AC current have already been proposed. 1,2,8,12,13 Though cathodic micro-discharges have never been observed within usual process conditions, the present study aims at defining suitable conditions to promote cathodic micro-discharges and at studying the main characteristics of these micro-plasmas.
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